The range of colors that a display device can print is defined as the color gamut of the device. The color gamut for a printer is different from that of a monitor. One reason for this is that the color gamut for a monitor is represented in digital red, green and blue (i.e. RGB) values which control the intensity of the monitor color phosphors producing a range of hues (e.g. red, green, blue, cyan, magenta etc.), lightnesses (i.e. how much white or black in the color) and saturations (i.e. how bright or intense the color is). In contrast, the printer is limited to the number of colors it can produce using its available inks arid their associated hues, lightnesses and saturations by a different process. Printer gamuts are often smaller than monitor gamuts. The colors that are the same for both gamuts are said to be in-gamut colors, whereas colors that are in one gamut but not the other are defined as out-of-gamut colors.
Color matching is a method that is used to match colors displayed by a source device (such as a monitor) to colors displayed by a destination device (such as a printer). Generally, the source device is defined as the device transmitting digital color information in the form of a digital color value and the destination device is the device receiving the color value. One manner in which color matching is performed is by using a color matching algorithm to calculate corrected color values to be inserted into a look-up table. Once a color table in created, color values transmitted from a monitor (typically RGB color values) are used to index the corrected values stored in the table. These corrected values are then transmitted to the printer and are used to instruct the printer what color it should print. The problem with this method is that creating the look-up tables is a very time intensive process due to the complexity of the algorithms used to generate the values to be stored in the look-up tables. In addition, the look-up tables generally take up large amounts of memory space.
Another problem with this type of color matching method is that different display devices often display colors in an inherently different manner. For example, a display monitor displays color by illuminating phosphor dots on its screen. The resulting color is the additive combination of the light from the phosphors. In contrast, a printer displays color by depositing ink on paper. The color is achieved by the ink's subtraction of light from the viewing illumination. Because of this, colors displayed on a monitor are additive whereas colors displayed on a printer are subtractive. Consequently, when monitor color components (e.g. red, green, and/or blue) are added to generate a third color the intensity of the resultant color is more intense than the two original colors. However, when more than one color is printed on paper with ink, each color is subtracted from the white viewing illumination and the resultant printer color is darker than each of the two color components that created it. As a result, a color value used to display color on a monitor screen will generate a darker color on a printer.
This problem becomes even more evident in the case in which a single component color value is used to display a color on a monitor and a multiple component color value is used to display a matching color on a printer. For instance, the monitor typically only needs to use a single component to display a fully saturated red, however in order for the printer to match this color, a color matching algorithm would often indicate that the printer needs to use two or three components, magenta, yellow and possibly black, to obtain a color that matches the monitor. The reason for this is that red on the printer is produced with two components, magenta and yellow, and as a result, the color will be darker. The inverse of the above case is also true--a printer can print a given color with a single component whereas the monitor needs to use two or more components to display that same color. For instance, printers can typically print cyans and magentas with a single component whereas a monitor must mix two components to generate these colors. As a result, the mixed monitor color is often lighter than the printed one component ink color.
Still another problem with color matching occurs when a monitor color falls out of the gamut of the printer (i.e. out-of-gamut colors). In this case, color matching algorithms often attempt to choose a printer color having a lightness, hue, and saturation close to that of the monitor color its being matched to. However, it is hard to match all three of these color characteristics due to the above described color display disparities between the monitor and the printer. As a result, color matching algorithms are sometimes designed to provide the same hue and lightness, but not the same saturation. The reason this is done is that, to the viewer, the lightness of a picture provides most of the information of the scene when compared to the saturation. However, although this is true for photos, where intense colors are not as critical, it is not true in other graphic display applications in which intensity (i.e. saturation) is important.
Thus, what is desired is an alternative color matching method to the presently used color matching algorithm techniques.